It has been know a so-called tandem-type image forming apparatus in which photosensitive members corresponding to respective colors of yellow, magenta, cyan and black are provided in parallel. In the tandem-type image forming apparatus, developer images are formed on the respective photosensitive members substantially at the same time. In an image forming apparatus adopting a direct transfer system, a conveyance belt is provided to contact each photosensitive member and developer images on the respective photosensitive members are transferred on a recording sheet while being color-overlapped, which is conveyed by the conveyance belt. As a result, a color image is formed on the recording sheet.
For example, JP-A-2009-3377 describes an image forming apparatus configured such that developer remaining on the photosensitive members is temporarily accumulated on rollers contacting surfaces of the photosensitive members, respectively, and the developer accumulated on the rollers is discharged to the conveyance belt through the photosensitive members after a printing operation is completed, and the like. Accordingly, the image forming apparatus is provided with a cleaning unit to remove developer from the conveyance belt.
The cleaning unit has a primary cleaning roller that contacts the conveyance belt and a secondary cleaning roller that contacts the primary roller, for example. The primary and secondary cleaning rollers are respectively applied with a predetermined cleaning bias. A potential difference between the conveyance belt and the primary cleaning roller and a potential difference between the primary cleaning roller and the secondary cleaning roller transfer the developer on the conveyance belt to the primary cleaning roller and the secondary cleaning roller in corresponding order. Thereby, the developer is removed from the conveyance belt.
In addition to the developer, sheet powders originating from a recording sheet are adhered on the conveyance belt. The adhesion of sheet powders to the conveyance belt is caused during the conveyance of the recording sheet. Accordingly, in order to exclude an influence of the sheet powders on a quality of an image that is formed on the recording sheet, it is necessary to apply the cleaning bias to the primary and secondary cleaning rollers even during an operation for forming an image (image forming operation).
However, from a power consumption point of view, it is not preferable to continuously apply the cleaning bias to the primary and secondary cleaning rollers not only during the cleaning mode after the completion of formation of the image on the recording sheet and but also during the image forming operation.
In addition, in the image forming apparatus, the same bias is applied to the cleaning unit irrespective of operation modes. Accordingly, when the printing operation is continuously performed, in particular, frictional force is increased between the primary cleaning roller and the conveyance belt, so that abnormal noise may be generated. This is caused because the developer serving as lubricant between the primary cleaning roller and the conveyance belt is collected from the primary cleaning roller to the secondary cleaning roller and is thus reduced gradually.